Prophet
The idea of a "prophet" is very closely tied to the idea of the Bible itself, since the Bible was largely written by prophets. At a lower level, prophets represent people who teach from the Bible. At a higher level, they represent the Lord as He reveals himself through the Bible. Viewed in a abstract way, prophets represent the holy parts of the Bible themselves, and also represent doctrine drawn from the Bible. The reason we say "largely written by prophets" and "the holy parts of the Bible" is that not all of the books currently included in the Bible have a complete and continuous internal sense. Some -- like Job, Ruth, and Song of Solomon -- are wonderful literary pieces that got included, but which lack the systematic meanings for words and phrases. Others -- the Acts and Epistles, primarily -- are really doctrinal works, the first attempt by others to extract meaning from Jesus' life and words.
Say
As with many common verbs, the meaning of “to say” in the Bible is highly dependent on context. Who is speaking? Who is hearing? What is it about? Is it a command, a message, an apology, instruction? All these things enter into the meaning of “say.” In general, though, “saying” has to do with sharing truth at various levels -- from the most exalted power people can have to perceive the Lord's desires directly to the most basic of orders issued to people at their lowest.